This invention relates in general to advanced composite materials and, more specifically, to moisture-impervious coatings for such materials.
Advanced composite materials, which comprise very high strength fibers imbedded in a synthetic resin matrix, are coming into wide-spread use, especially in aerospace and sporting goods applications where high stiffness and a high strength-to-weight ratio are required. In many of these applications, dimensional stability of previously formed components is essential. Unfortunately, many of these composite materials exhibit significant dimensional instability when exposed to moisture, either directly as water or in high humidity environments. They also tend to contaminate high vacuum environments, such as space systems, due to outgassing.
A number of protective and moisture resistant coatings have been developed for various materials for different purposes. Synthetic resin coatings developed for moisture resistance are not truly impervious to moisture and tend to degrade in hostile environments. Combinations of resin coatings and electroplated metal films, such as are described by Robinson in U.S. Pat. No. 2,456,995 are resistant but not totally impervious to moisture, due apparently to "pinholes" or other paths through the coating. Similarly, sprayed metal coatings (such as are described by Luense in U.S. Pat. No. 1,261,141) and glow discharge deposited metal coatings (such as are described by Lindblom et al in U.S. Pat. No. 3,686,013) while moisture resistant are not totally impervious to moisture, apparently due to the fact that such coatings are slightly porous since they are composed of overlapping particles of metal which resist moisture only because of long vapor paths through thick coatings.
By forming a very thick metal coating by the above techniques, very high resistance to moisture can be obtained. However, such thick coatings add little to the strength of the substrate, while adding greatly to the weight. This would tend to defeat the purpose of the composite material, i.e., the high strength-to-weight ratio. This added weight is especially disadvantageous in aerospace applications.
Thus, there is a continuing need for a method of forming moisture impervious coatings over advanced composite materials without adding excessive weight to the composite article.